Sensitivity to deliberate sea salt seeding of marine clouds – observations and model simulations

Sea salt seeding of marine clouds to increase their albedo is a proposed technique to counteract or slow global warming. In this study, we first investigate the susceptibility of marine clouds to sea salt injections, using observational data of cloud droplet number concentration, cloud optical depth, and liquid cloud fraction from the MODIS (Moderate Resolution Imaging Spectroradiometer) instruments on board the Aqua and Terra satellites. We then compare the derived susceptibility function to a corresponding estimate from the Norwegian Earth System Model (NorESM). Results compare well between simulations and observations, showing that stratocumulus regions off the west coast of the major continents along with large regions in the Pacific and the Indian Oceans are susceptible. <br><br> We then carry out geo-engineering experiments with a uniform increase of 10⁻⁹ kg m⁻² s⁻¹ in emissions of sea salt particles with a modal radius of 0.13 μm. The increased sea salt concentrations and the resulting change in marine cloud properties lead to a globally averaged forcing of −4.8 W m⁻² at the top of the atmosphere, more than cancelling a doubling of CO₂ concentrations. The forcing is large in areas found to be sensitive by using the susceptibility function, confirming its usefulness as an indicator of where to inject sea salt for maximum effect. <br><br> Results also show that the effectiveness of sea salt seeding is reduced because the injected sea salt provide a large surface area for water vapor and gaseous sulphuric acid to condense on, thereby lowering the maximum supersaturation and suppressing the formation and lifetime of sulphate particles. In some areas, our simulations show an overall reduction in the CCN concentration and the number of activated cloud droplets decreases, resulting in a positive globally averaged forcing.